Fluorescent ballast tests and teardowns - with schematics

One of these lights will last for over 50 years. Spoilers - it's not the electronic one. But the downside is that it is also massively less efficient. The ballast dissipates almost as much power as the tube does. A rough calculation shows that it's mainly due to the winding resistance of the ballast at the rated current. (operating current 0.16A times winding resistance of 190 ohms = 30.4V dropped resistively. 30.4V times 0.16A = 4.9W of heat!) Not always a bad thing in outdoor fixtures.

In this video I compare the electrical characteristics and then reverse engineer the circuitry inside each light. It would have been nice to measure the average current through the tube on each circuit, but that would be complicated greatly by the weird current waveforms and the very high frequency of the electronic ballast.

The electronic ballasts are kinda interesting. They started off quite complex and unreliable. But by the end they were incredibly optimised and reliable - even with the baking hot temperatures in compact fluorescent bulb bases.

Another factor affecting the frequency of operation of the electronic ballast is the saturation of the feedback toroid. When it can't couple more current to the transistor coils they will turn off and the other transistor will be turned on as the magnetic coupling reverses.

I'm not so sure about the use of a polarised electrolytic capacitor on the base circuit of the lower transistor. Its polarity seems unintuitive, but is a common thing in these self-starting circuits.



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